The over-production of reactive oxygen species (ROSs), such as H2O2, O2−, and hydroxyl radicals, has been associated with several lethal and debilitating health conditions. Heightened oxidative damage to proteins and other biomolecules has been observed in the biopsies and post-mortem examinations of patients suffering from a wide variety of cardiovascular and neurological diseases. Understanding the roles that ROSs play in the progressions of these and other conditions requires probes that can monitor their production and traffic within biological systems. Currently, most sensors capable of directly detecting ROSs rely on either fluorescent or luminescent outputs. Although these probes provide high spatial resolution, the short wavelengths of light needed to excite the reporter make imaging activity in samples other than thin tissues and cell cultures difficult.
Magnetic resonance imaging (MRI), conversely, uses radio-frequency photons to excite the hydrogen nuclei in water molecules and can be used to visualize tissues and organs deep within thicker biological samples. Therefore, magnetic resonance imaging (MRI) is commonly used as a non-invasive diagnostic tool in medicine. Since the bulk of the body's 1H nuclei are from water molecules, MRI often differentiates tissues on the basis of their water content. Contrast agents are often added to accelerate the relaxation of the excited 1H nuclei back to the ground state; this increases the amount of RF radiation that can be absorbed, thereby enhancing the contrast between water-rich and water-deficient regions. Either spin-lattice (T1) or spin-spin (T2) relaxation times can be monitored, but small molecule contrast agents generally induce larger changes in T1. The use of a responsive contrast agent, which exhibits a different relaxivity (r1) upon exposure to an analyte, can allow researchers to visualize a biochemical process within a whole-body subject in concert with clinically approved MRI instrumentation.
Most small molecule MRI contrast agents shorten the longitudinal relaxation times (T1) of excited protons, allowing sharper contrast between regions with high and low water contents. The ability to accelerate these relaxations defines the T1-weighted relaxivity (r1) of the contrast agent. A molecule that displays a different r1 value upon the addition of an analyte can serve as a sensor when monitored by MRI.
Several such MRI contrast agent sensors have been developed, but few have been directed towards imaging oxidative activity. The probes capable of detecting oxidants often either require a co-analyte or display a similar response to O2 or another analyte. Most redox-responsive contrast agent probes with mononuclear metal centers function via changes in the oxidation state of the metal, with the more paramagnetic species having the greater r1. Caravan and co-workers, for instance, reported a series of manganese-containing contrast agents capable of switching between the +2 and +3 oxidation states through reactions with glutathione and H2O2. (Loving, G. S.; Mukherjee, S.; Caravan, P. J. Am. Chem. Soc. 2013, 135, 4620-4623.) Morrow's group recently reported an oxygen-sensitive cobalt complex that toggles between paramagnetic +2 and diamagnetic +3 oxidation states; it should be noted that this contrast agent operates through a PARACEST mechanism, rather than changes in T1. (Tsitovich, P. B.; Spernyak, J. A.; Morrow, J. R. Angew. Chem. Int. Ed. 2013, 52, 13997-14000.)
An alternative strategy is to couple the change in the MRI properties to a change in the oxidation state of the ligand, rather than the metal. The research groups of Sherry, Louie, and Pagel used this approach to develop lanthanide complexes that activate either upon reduction by β-NADH or ascorbic acid or upon oxidation by mixtures of NO and O2 or singlet oxygen. (Ratnakar, S. J.; Viswanathan, S.; Kovacs, Z.; Jindal, A. K.; Green, K. N.; Sherry, A. D. J. Am. Chem. Soc. 2012, 134, 5798-5800; Tu, C.; Nagao, R.; Louie, A. Y. Angew. Chem. Int. Ed. 2009, 48, 6547-6551; Liu, G.; Li, Y.; Pagel, M. D. Magn. Reson. Med. 2007, 58, 1249-1256; Song, B.; Wu, Y; Yu, M.; Zhao, P.; Zhou, C.; Kiefer, G. E.; Sherry, A. D. Dalton Trans. 2013, 42, 8066-8069.)
Recently, a mononuclear manganese complex capable of directly detecting H2O2; notably, the complex lacks a chemical response to O2 was reported. (Yu, M.; Beyers, R. J.; Gorden, J. D.; Cross, J. N.; Goldsmith, C. R. Inorg. Chem. 2012, 51, 9153-9155.) However, upon oxidation, the mononuclear complexes irreversibly couple into binuclear Mn(II) species. The reaction with H2O2 decreases the T1-weighted relaxivity per manganese ion; that the response is a reduction in contrast enhancement limits the probe's ability to resolve different levels of H2O2. Needless to say, a better MRI contrast agent for detecting H2O2 or other oxidants is desired.
The objective of this invention is to develop metal complexes with ligands that are redox-responsive to oxidants, preferably reactive oxygen species.
The other objective of this invention is to develop a MRI contrast agent that is not only capable of directly detecting a reactive oxygen species, but also lacks a chemical response to O2 and displays an increase in its T1-weighted relaxivity upon the ligand's oxidation.
Another objective of this invention is to develop a pharmaceutical agent that is capable of being both a MRI contrast agent and a therapeutic agent for reducing reactive oxygen species in a subject, such as in a mammal or human.
Other objects, advantages, and features of the present invention will become apparent from the following specification taken in conjunction with the accompanying examples, figures, and drawings.